This invention relates in general to circuits for charging batteries and in particular to a circuit with a switch and a control device for charging a battery, which is combined with a capacitor.
Referring now to the drawings, there is illustrated in FIG. 1 a known circuit 10 for charging a battery 12 combined with a capacitor 16. The battery 12 has a first battery terminal 13 and a second battery terminal 14. The capacitor has a first capacitor terminal 17 and a second capacitor terminal 18.
Preferably, the second battery terminal 14 is connected to ground, as indicated at 55.
The circuit 10 includes a blocking diode 52. The blocking diode 52 has a cathode 53 and an anode 54. The anode 54 is connected to the first battery terminal 13.
Also shown in FIG. 1 is a power supply 20, as indicated by dashed lines. The power supply 20 includes a motor-generator 24, and a power converter 28. The motor-generator 24 is an AC motor-generator. The motor-generator 24 has first and second motor-generator terminals 25 and 26.
The power converter 28 includes a rectifier, for converting the AC power from the motor-generator 24 to DC power. The power converter 28 has first and second input ports 29 and 30. The power converter 28 has first and second output ports 21 and 22. The first motor-generator terminal 25 is connected to the first input port 29. The second motor-generator terminal 26 is connected to the second input port 30. The motor-generator 24 is thus connected to the power converter 28.
The circuit 10 further includes a switch 32. The switch 32 has first a first switch terminal 33 connected to the first output port 21. The switch 32 has a second switch terminal 34 connected to the first battery terminal 13. The switch 32 also has a control terminal 35. The switch 32 is responsive to a control signal applied to the control terminal 35 to change between a conducting state that allows current flow between the first and second switch terminals 33 and 34 and a non-conducting state that prevents current flow between the first and second switch terminals 33 and 34. While the switch 32 is shown as an electrical device, it will be appreciated that the switch 32 may be a mechanical device or any other suitable switch.
Typically, a switch control line 48 connects the power converter 28 to the switch control terminal 35.
The power converter 28 is operable to generate a switch control signal to switch the switch 32 to the conducting state to charge the battery 12 when the power converter 28 is on and to switch the switch 32 to the non-conducting state when the power converter 28 is off. The switch control signal is transmitted to the switch 32 via the switch control line 48.
The first capacitor terminal 17 and the cathode 53 are connected to the first output port 21. The second capacitor terminal 18 and the second battery terminal 14 are connected to the second output port 22. The first battery terminal 13 being connected to the anode 54, the battery 12 and capacitor 16 are, thus, connected in parallel when either the diode 52 or the switch 32 is in the conducting state.
The capacitor 16 is suitable to provide energy for high current surges and the battery 12 is to provide energy during longer duration loads, as compared to the capacitor 16. When sufficient energy has been drained out of the capacitor 16, the voltage of the capacitor 16 will drop to the same voltage as the battery 12, and the diode 52 will change to a conducting state. When the diode 52 is in the conducting state, the battery 12 and the capacitor 16 will supply energy simultaneously. However, most of the energy is then supplied from the battery 12. When energy has been drained out of the battery 12, the battery 12 will to be recharged. The main purpose of the capacitor 16 is to shield the battery 12 from large current pulses, which for (+) IL is accomplished if and only if the voltage of the capacitor 16 is higher than the voltage of the battery 12. The capacitor 16 shields the battery 12 for all (−) IL as long as the switch 32 is open.
In operation, to charge the battery 12, the switch 32 is closed, or in a conducting state, and the power supply 20 provides power to the battery 12. Conversely, when the switch 32 is open, or in a non-conducting state, the power supply 20 does not supply power to the battery 12.
During charging, the voltage of the battery 12 and the voltage of the capacitor 16 are almost identical. Thus, a (+) IL pulse that may occur during charging may conduct through the diode 52 and the pulse will be shared between the capacitor 16 and the battery 12. Thus, during charging the capacitor 16 cannot shield the battery 12 from (+) IL current pulses, the main reason for combining the capacitor 16 with the battery 12. In practice, most of the (−) IL pulses will be much larger than the charging current, but the switch 32 will only be rated for the charging current. Therefore, during charging, the power supply 20 must detect the start of a (−) IL pulse and open the switch 32 in order to protect the switch 32. If the switch 32 is a mechanical device, it also will conduct (+) IL current and, therefore, also must be opened before the start of a (+) IL pulse.